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Effects of joint properties and geometry on fractured rock strength using the distinct element method

Distinct element methods are used extensively in modelling the mechanical responses of highly jointed rock masses over a wide range of scales and in-situ stress conditions. An important consideration in the design of these models is the selection of appropriate geometries for the joints. These joints both define the rock mass structure and provide for shear and tensile failure of the material. Joint geometry thus affects the overall strength of the rock as well as introducing anisotropy to the material. In many large scale problems it is challenging to model joint geometry on reality, either because of difficulties in determining the geometry or computational constraints in simulating so many joints. In order to better understand the effects of unknown joint patterns on rock behaviour a number of numerical simulations have been performed. The commercially available 2D Universal Distinct Element Code (UDEC) has been used to model triaxial compression tests on several large scale cylindrical rock masses, using axial symmetry to simulate the physical 3D system. Simulations were performed across a range of deterministic and random patterns, including statistically distributed patterns based on real rock mass data. The effects of element size, joint stiffness and joint friction angle were also investigated. Changes in joint pattern are shown to give considerable difference in both the elastic deformation and failure behaviour of the rock. Regular, square based patterns are shown to give strong, brittle materials while more angled geometries tend to give weaker, ductile behaviour. Rock mass cohesion and friction angle are shown to be particularly dependent on joint geometry. In contrast, joint stiffness and friction properties are shown to have a relatively minor influence on rock mass properties.

Author(s):

Benjamin Roullier    
University of Nottingham
United Kingdom

Paul Langston    
University of Nottingham
United Kingdom

Xia Li    
University of Nottingham
United Kingdom